Method for producing directionally solidified nickel base alloy

ABSTRACT

A METHOD OF MAKING A CAST BODY FROM A NICKEL BASE ALLOY HAVING IMPROVED STRESS RUPTURE STRENGTH IS PROVIDED. THE METHOD INVOLVES FORMING A NICKEL BASE ALLOY CONTAINING A CAREFULLY CONTROLLED COMBINATION OF GAMMA PRIME FORMING ELEMENTS AL, TI AND TA ALONG WITH A CRITICAL AMOUNT OF CR, POURING THE MOLTEN ALLOY INTO A MOLD AND DIRECTIONALLY SOLIDIFYING IT INTO A CAST BODY, REMOVING THE LOW MELTING SEGREGATED PHASES FROM THE DIRECTIONALLY SOLIDIFIED CAST BODY BY A DISSOLUTION HEAT TREATMENT TO PERMIT THE SEGREGATED PHASES TO DISSOLVE IN SOLID SOLUTION WITHOUT INCIPIENT MELTING, AND COOLING THE CAST BODY TO ROOM TEMPERATURE.

Jan. 1, 1974 WALKER ETAL 3,783,032

METHOD FOR PRODUCING DIRECTIONALLY SOLIDIFIED NICKEL BASE ALLOY Filed July 31, 1972 2 Sheets-Sheet 1 Jan; 1, 1974 J L R ETAL 3,783,932

METHOD FOR PRODUCING DIRECTIONALLY SOLIDIFIED NICKEL BASE ALLOY 2 Sheets-Sheet 2 Filed July 31, 1972 w N Q vv m 2 Q m N 9 l M a m K Q I, i, Q

United States Patent 3,783,032 METHOD FOR PRODUCING DIRECTIONALLY SOLIDIFIED NICKEL BASE ALLOY James L. Walker, Schenectady, and Thomas F. Sawyer,

Ballston Lake, N.Y., assignors to General Electric Com- Filed July 31, 1972, Ser. No. 276,752 Int. Cl. C221? 1/10 US. Cl. 148--3 4 Claims ABSTRACT OF THE DISCLOSURE A method of making a cast body from a nickel base alloy having improved stress rupture strength is provided. The method involves forming a nickel base alloy; containing a carefully controlled combination of gamma, prime forming elements Al, Ti and Ta along with a critical amount of Cr, pouring the molten alloy into a mold and directionally solidifying it into a cast body, removing the low melting segregated phases from the directionally solidified cast body by a dissolution heat treatment to permit the segregated phases to dissolve in solid solution without incipient melting, and cooling the cast body to room temperature.

This invention relates to nickel base superalloys Which are particularly useful in gas turbine engines. In designing these engines for jet aircraft, it is a requirement that the alloy have good long time rupture properties and have hot corrosion resistance. Thus substantial efforts have been made to improve the properties of the nickel base alloys.

The copending application of Aldred et al. Ser. No. 261,238, filed June 9, 1972, and assigned to the assignee of the present invention describes a nickel base alloy for the manufacture of cast articles. The alloy has an improved combination of stress rupture strength and resistance to sulfidation along with enhanced stability to the formation of detrimental phases, i.e. sigma and eta, primarily through a carefully controlled combination of the gamma prime forming elements Al, Ti and Ta along with a critical amount of Cr. As is disclosed by US. Pat. 3,260,505, directional solidification of nickel base superalloys when cast into gas turbine and gas contacting blades yields advantages in tensile properties, creep rupture properties, casting properties and thermal shock resistance.

With advances in the metallurgical technology for nickel bases superalloys, such as those alloys used in the present invention, the alloys capabilities have been pushed to the practical safe maximum by increasing the amount of the gamma prime formers, i.e. Al, Ti and Ta. Unfortunately, there is a corresponding increase in the tendency to form segregate phases which arise because of the nonequilibrium conditions present during solidifica-. tion of the alloys. These phases are observed to be distributed interdentritically within a cast body. The relatively low solidification rates characteristic of directionally solidified superalloy bodies results in the formation of large islands of such nonequilibrium phases. Since these islands are aligned dendritically by the directional solidification process the possibility exists for oriented weak zones within the directionally solidified superalloy body. In our copending application, Ser. No. 100,053, filed Dec. 21, 1970, now abandoned, we describe a method of heat treating nickel base alloys to dissolve low melting phases. However, the extent of improvement in the alloy properties is dependent on the alloy com position and casting method.

Quite surprisingly, we have now found that for a, special class of nickel base alloys containing large amounts of gamma prime ingredients in which the mechanical 3,783,032 Patented Jan. 1, 1974 properties have been improved by directional solidification, further significant improvements in these properties can be obtained by subjecting the cast body to a dissolution heat treatment. This heat treatment imparts a significant increase in the stress-rupture parameters which is of particular importance in forming gas turbine engines.

In accordance with the present invention, we have discovered an improved method of making a cast body of a nickel base superalloy by forming a nickel base alloy containing a carefully controlled combination of gamma prime forming elements vAl, Ti and Ta along with a critical amount of Cr, pouring the molten alloy into a mold and directionally solidifying it into a cast body, removing the low melting segregated phases from the directionally solidified cast body by a dissolution heat treatment to permit the segregated phases to dissolve in solid solution without incipient melting, and cooling the cast body to room temperature.

The alloy which is treated by the process of the present invention is described in the Aldred et al. US. patent application cited above which is incorporated herein by reference. The alloy consists essentially in weight percent of:

Additionally, the alloy is further characterized in that the sum of Al and Ti is a minimum of 8.1%, the sum of Al, Ti, Ta and 'Nb is in the range of 11.7-12.5%, the sum of Ta and Nb is in the range of 3.6-4.4% and the sum of Mo and W is a minimum of 8.5%. The preferred composition is set forth in Example 1 hereinbelow.

The invention is more clearly understood from the following description taken in conjunction with the accompanying drawing in which:

FIG. 1 is a photomicrograph (2000 of a cross section of the preferred nickel base alloy subjected to the Standard Heat Treatment;

FIG. 2 is a photomicrograph (2000 of a cross section of the preferred nickel base alloy subjected to the novel Dissolution Heat T reatment; and

FIG. 3 is a graphic representation of the stress-rupture parameters of the preferred nickel base alloy subjected to the Standard Heat Treatment as compared to that subjected to the Dissolution Heat Treatment."

The alloy described hereinabove is poured into a mold and directionally solidified into a cast body by a technique which would be obvious to a person skilled in the art. Upon directionally solidifying the alloy, an elongated columnar macro-grain structure is obtained with substantially unidirectional crystals aligned substantially parallel to the direction of maximum heat flow.

Thereafter, the low melting segregate phases are removed from the directionally solidified cast body by a Dissolution Heat Treatment to permit the segregated phases to dissolve in solid solution without incipient melting. During this step, the cast body is heated to a temperature below the incipient melting temperature of the segregated phases for a time sulficient to dissolve a portion of the lower melting phases in the matrix whereby the incipient melting temperature of the resulting mass is increased. Thereafter, the temperature is gradually raised, but maintained below the increased incipient melting temperature, until the low melting segregated phases form a substantially homogeneous mass with the matrix.

The initial temperature of the dissolution heat treatment is determined by heating the alloy to a predetermined temperature, and removing and microscopically examining it for incipient melting. If incipient melting has occurred, then the temperature is too high and must be decreased. On the other hand, if incipient melting has not occurred, the predetermined temperature may be increased. A starting temperature for the dissolution heat treatment may thus be selected which is just below the temperature at which incipient melting of the lowest melting phase occurs. The heat treatment then proceeds at increasing temperatures up to a point below the temperature at which the solid solution of the alloy melts, but proceeding at a rate sufiiciently slow to avoid melting the low melting phases on subsequent temperature increases, until all the low melting phases are substantially solutionized.

The novel heat treatment may be stepwise procedure with holding periods, e.g. 18 F. for 4 hours, or it may be a gradual continuous process, e.g. at a rate of 2.5 C. per hour. However, the values given are merely for purposes of illustration. For all practical use, the rate should be as rapid as possible and this rate may be determined routinely by a person skilled in the art.

After cooling to room temperature, the dissolution heat treatment can then be followed by a precipitation hardening heat treatment.

Our invention is further illustrated by the following examples:

EXAMPLE I A preferred alloy having the following nominal composition in weight percent was vacuum melted in a magnesium oxide crucible using standard melting procedures.

Composition-- Weight percent Aluminum 4.25 Chromium 9.25 Titanium 4.0 Cobalt 10.0 Molybdenum 2.0 Tungsten 7.0 Tantalum 3.7 Carbon 0.17 Boron 0.015 Zirconium 0.05

The molten alloy was cast in a shell mold containing a number of cylindrical cavities approximately 4" in diameter and 6" long and cooled to room temperature. The morphology of the grains in the rods thus formed was typically equiaxed.

Directional solidification was performed at 2" per hour by a standard Bridgman technique. The diameter ingots were placed within a Vs" inside diameter, highpurity alumina crusible positioned on a water-cooled copper plate. This assembly, inside a glass sleeve, was placed under a flowing argon atmosphere. Power was supplied by induction to a susceptor surrounding the crucible causing the alloy to melt. A temperature gradient was thus established between the hot liquid alloy and the cold copper-chill plate. The ingot and the crucible were then lowered out of the hot zone at a rate of 2" per hour by a lowering device attached to the chill plate. Due to the steep thermal gradient (-100 C./cm.), the grains, nucleated at the chill plate, grew in a direction normal to the chill plate producing a number of elongated grains in the ingot.

Each of the ingots was cut into 1%" lengths and cylinders were electromachined therefrom. Mechanical test specimens were machined to produce test bars with the long axis parallel to the longitudinal direction in each ingot.

The alloy samples to be tested were heat treated in vacuum using the following schedule which is designated as the Standard Heat Treatment.

Step Temperature Time A Heat to 2,102 F Hold for 2 hours. B Heat to 2,228 F Hold for 1 hour. 0 Helium quench to room temperature-- D Heat to 1,994 F Hold for 4 hours. E Helium quench to room temperature F Heat to 1,652 F Hold for 16 hours. G Helium quench to room temperature Following the procedure of Example I, the preferred alloy composition was vacuum melted, cast, directionally solidified and then machined into test specimens. Thereafter, the alloy samples to be tested were heat treated in vacuum using the following schedule which is designated as the Dissolution Heat Treatment.

Step Temperature Time A Heart to 2,05% F Hold for 4 hours. B Raise temperature stepwise in 18 Hold for 4 hours at increments up to 2,282 F. each increment.

Helium quench to room temperature.-

D Heat to 1,99 F Hold for 4 hours. E. Helium quench to room temperatur F. Heat to 1,652 F Hold for 16 hours. G Helium quench to room temperature.

The Dissolution Heat Treatment varies from the Standard Heat Treatment by steps A, B and C. The remaining steps in both procedures are similar.

The mechanical test samples were then subjected to the standard test to determine the stress rupture properties. The results are illustrated in FIG. 3 which shows the curve of the stress-rupture parameter for the samples subjected to the Dissolution Heat Treatment.

Similar to Example I, a metallographic sample was cut in cross section and a photomicrograph (2000 magnification) is shown in FIG. 2. It is readily apparent that the nonequilibrium phases which were present in FIG. 1, have now been removed. The large island in the upper left-hand corner is identified as a carbide.

A comparison between the samples subjected to the Standard Heat Treatment and the Dissolution Heat Treatment clearly indicates in FIG. 3 that the Dissolution Heat Treatment produces a substantially superior product. This phenomena is explained by the fact that the nonequilibrium phases shown in FIG. 1 have been removed by the novel Dissolution Heat Treatment as shown in FIG. 2. The presence of these nonequilibrium phases are considered to weaken the cast alloy bodies.

Thus, it may be concluded that by combining the processes of direction solidification and dissolution heat treatment it is possible to produce a nickel base superalloy body with properties superior to those of a normal directionally solidified alloy.

It will be appreciated that the invention is not limited to the specific details shown in the illustrations and that various modifications may be made within the ordinary skill in the art without departing from the spirit and scope of the invention.

We claim: 1. An improved method of making a cast body of a nickel base superalloy comprising the steps of:

(a) forming an alloy consisting essentially in weight percent of Ingredient- Weight percent Carbon 0.05-0.35 Chromium 8-10 Titanium 3.3-4.3 Boron 0.01-0.03 Aluminum 3.8-4.8 Tungsten 6-8 Molybdenum 1-3 Cobalt 8-15 Tantalum 3.6-4.4 Zirconium 0.-01 Niobium 0-2 Hafniurn 0-2 Nickel Balance wherein the sum of Al and Ti being a minimum of 8.1%, the sum of Al, Ti, Ta, and Nb being in the range of 11.7-12.5 the sum of Ta and Nb being in the range of 3.64.4%, and the sum of Mo and W being a minimum of 8.5%,

(b) pouring said alloy into a mold at a temper-ature above its melting point,

(c) directionally solidifying the alloy whereby an elongated columnar macro-grain structure is obtained with substantially unidirectional crystals aligned substantially parallel to the direction of maximum heat flow,

(d) removing the low melting segregated phases from the directionally solidified cast body by a dissolution heat treatment to permit the segregated phases to dissolve in solid solution without incipient melting, and

(e) cooling the cast body to room temperature.

2. The method of claim 1, wherein the low melting segregated phases are removed by heating the body to a temperature below the incipient melting temperature of said phases and for a time sufficient to dissolve a portion of the lower melting segregated phases in the matrix whereby the incipient melting temperature of the resulting mass is increased, and gradually raising the temperature of the heat treatment below the increased incipient melting temperature until the low melting segregated phases form a substantially homogeneous mass with the matrix.

3. The method of claim 2, wherein said alloy consists essentially in weight percent of about:

4. The method of claim 3, wherein the heat treatment comprises heating the cast body to a temperature of about 2084 F. and then raising the temperature stepwise up to a maximum of about 2282 F.

References Cited UNITED STATES PATENTS 3,260,505 7/ 1966 Ver Snyder -171 RICHARD O. DEAN, Primary Examiner US. Cl. X.R. 

